ci: avoid always running build/test jobs

Introduce a trigger job with rules that will allow it to always
run when the MR is assigned to Marge and the final tests will be
run before being merge.

This means that Post-merge pipelines and MR pipelines that are
not assigned to Marge, will require a manual run of the trigger
job before the build/test will execute.

This should help easy the load on the gitlab runners.

Related to gstreamer/gstreamer-rs#417

Close gstreamer/gstreamer-rs#401

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1155>

.gitlab-ci.yml

@@ -60,6 +60,7 @@ variables:
     0.18=0.18
 
 stages:
+  - "trigger"
   - "container-base"
   - "container-final"
   - "lint"
@@ -67,8 +68,29 @@ stages:
   - "extras"
   - "deploy"
 
+# This is an empty job that is used to trigger the pipeline.
+trigger:
+  image: alpine:latest
+  stage: 'trigger'
+  variables:
+    GIT_STRATEGY: none
+  script:
+    - echo "Trigger job done, now running the pipeline."
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "schedule"
+    # If the MR is assigned to the Merge bot, trigger the pipeline automatically
+    - if: '$CI_MERGE_REQUEST_ASSIGNEES == "gstreamer-merge-bot"'
+    # Require explicit action to trigger tests post merge
+    - if: '$CI_PROJECT_NAMESPACE == "gstreamer" && $CI_COMMIT_BRANCH == "main"'
+      when: 'manual'
+    # When the assignee isn't the merge bot, require an explicit action to trigger the pipeline
+    # to avoid wasting CI resources
+    - if: '$CI_MERGE_REQUEST_ASSIGNEES != "gstreamer-merge-bot"'
+      when: 'manual'
+      allow_failure: false
 
 .debian:11:
+  needs: []
   variables:
     FDO_DISTRIBUTION_VERSION: 'bullseye-slim'
   before_script:
@@ -273,6 +295,8 @@ test msrv:
     - '.cargo test'
     - .img-msrv
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-msrv'
       artifacts: false
 
@@ -281,6 +305,8 @@ test stable:
     - '.cargo test'
     - .img-stable
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-stable'
       artifacts: false
 
@@ -292,6 +318,8 @@ test stable all-features:
     - '.cargo test'
     - .img-stable
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-stable'
       artifacts: false
 
@@ -302,6 +330,8 @@ test nightly:
     - '.cargo test'
     - .img-nightly
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-nightly'
       artifacts: false
 
@@ -315,6 +345,8 @@ test nightly all-features:
     - '.cargo test'
     - .img-nightly
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-nightly'
       artifacts: false
 
@@ -374,6 +406,8 @@ test stable sys:
     - '.cargo test sys'
     - .img-stable
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-stable'
       artifacts: false
 
@@ -382,6 +416,8 @@ test msrv sys:
     - '.cargo test sys'
     - .img-msrv
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-msrv'
       artifacts: false
 
@@ -390,6 +426,8 @@ test nightly sys:
     - '.cargo test sys'
     - .img-nightly
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-nightly'
       artifacts: false
 
@@ -418,6 +456,8 @@ clippy:
   variables:
     CLIPPY_LINTS: -D warnings -W unknown-lints
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-stable'
       artifacts: false
   script:
@@ -493,6 +533,8 @@ coverage:
     - .img-stable
   stage: 'extras'
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'build-stable'
       artifacts: false
   variables:
@@ -596,6 +638,7 @@ pages:
 
 .windows rust docker build:
   stage: 'container-final'
+  needs: []
   variables:
     # Unlike the buildah/linux jobs, this file
     # needs to be relative to windows-docker/ subdir
@@ -668,12 +711,16 @@ windows rust docker msrv:
 test windows msrv:
   image: $WINDOWS_RUST_MINIMUM_IMAGE
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'windows rust docker msrv'
       artifacts: false
   extends: '.msvc2019 build'
 
 test windows stable:
   needs:
+    - job: 'trigger'
+      artifacts: false
     - job: 'windows rust docker stable'
       artifacts: false
   image: "$WINDOWS_RUST_STABLE_IMAGE"